Anti-fungal compounds produced by plants in response to fungal infection are termed phytoalexins. During a previous reporting period, we identified the major phytoalexin of Arabidopsis thaliana, a thiazole containing indole derivative, arabilexin (= camelexin). This project has now focused on identifying minor phytoalexins produced by this plant. We have tentatively identified a second component, the N-methyl derivative of arabilexin. The identification of these components in Arabidopsis will allow molecular genetic studies of the regulation of these inducible defense compounds, if mutants can be identified with altered phytoalexin expression. However, an important prerequisite for these studies is the characterization of the biosynthetic steps leading to the final endproducts. Using radio- and stable isotope labeling, intermediates have been isolated and characterized. Mass spectrometry (EI- and FAB-MS, as well as GC-MS) has played a key role in this project.